I had a similar problem one time, it was associated with Microsoft Word. I would be working in it and suddenly the computer would restart itself. The problem was not caused by a virus, it was probably corrupt files. I had to folow some simple instructions found in Microsoft Knowledge base. http://support.microsoft.com/search/preview.aspx?scid=kb;en-us;Q302602
CAUSE
The following files may be corrupted:
Mso9.dll
Winword.exe
RESOLUTION
To resolve this issue, follow these steps:
On the taskbar, click Start , point to Find , and then click Files and Folders .
On the Name & Location tab, type mso9.dll in the Named box, and then click Find Now .
In the list of files found, right-click mso9.dll , and then click Rename . Type mso9.old , and then press ENTER.
Repeat steps 1 through 3 to find the Winword.exe file and rename it to Winword.old. Close the Find dialog box.
On the taskbar, click Start , point to Settings , and then click Control Panel .
In Control Panel, double-click Add/Remove Programs .
On the Install/Uninstall tab, click Microsoft Office 2000 in the list of installed programs, and then click Add/Remove .
In the Maintenance Mode dialog box, click Repair Office .
In the Reinstall or Repair dialog box, click Repair errors in my Office installation , and then click Finish .
Try this and see what happens. If the problem remains, search the Microsoft Knowledge Base for the problem and solution to it.